Strain wave gears, also known as harmonic drives, are unique gearing systems that can provide high reduction ratios, high torque-to-weight and torque-to-volume ratios, near-zero backlash (which can mitigate the potential wearing of the components), and a host of other benefits. Typically, strain wave gears include an elliptical wave generator that is fitted within a flexspline such that the flexspline conforms to the elliptical shape of the wave generator; this arrangement also typically includes a set of ball bearings that allow the flexspline to rotate about the central axis of the elliptical shape relative to the wave generator. The flexspline is generally disposed within a ring-shaped circular spline, where the flexspline includes a set of gear teeth along its outer, elliptically shaped, perimeter that engage with gear teeth disposed along the inner circumference of the rim-shaped circular spline. Typically, the flexspline has fewer teeth than the circular spline. Notably, the flexspline is made of a flexible material such that when gear teeth of the flexspline and circular spline are engaged, the wave generator can rotate relative to the circular spline in a first direction, and thereby cause the deformation and associated rotation of the flexspline in a second opposite direction. Normally, an input torque is provided to the wave generator, and the flexspline generates a resulting output torque. Typically, the rate of rotation of the wave generator is much greater than the rate of rotation of the flexspline. Thus, strain wave gears can achieve high reduction ratios relative to gearing systems and can do so in a smaller form factor.
Note that in some alternative arrangements, the flexspline is held fixed, and the circular spline is used to provide an output torque.
As can be inferred, the operation of a strain wave gear is particularly nuanced and relies on a very precisely engineered gearing system. For example, the geometries of the constituent parts of strain wave gears must be fabricated with extreme accuracy in order to provide the desired operation. Moreover, the strain wave gear components must be fabricated from materials that can provide for the desired functionality. In particular, the flexspline must be flexible enough to withstand high-frequency periodic deformation, while at the same time being strong enough to accommodate the loads that the strain wave gear is anticipated to be subjected to.
Because of these constraints, heritage strain wave gears have largely been fabricated from steel, as steel has been demonstrated to possess the requisite materials properties, and steel can be machined into the desired geometries. However, the machining of steel into the constituent components can be fairly expensive. For example, in many instances, steel-based strain wave gears can cost on the order of $1,000 to $2,000 largely because of the expensive manufacturing processes.
In some instances, harmonic drives are fabricated from thermoplastic materials. Thermoplastic materials (e.g. polymers) can be cast (e.g. via injection molding processes) into the shapes of the constituent components, and thereby circumvent the expensive machining processes that are typically implemented in manufacturing steel-based strain wave gears. However, strain wave gears fabricated from thermoplastics may not be as strong as strain wave gears fabricated from steel.